Mechanical scanning for ultrasonic inspection of regularly shaped objects, such as for example flat or slightly curved objects, is well documented and widely used. An object is typically mapped by software using a grid, for example a rectangular grid, in which a length and a width of each rectangular pixel of the grid is identical. The grid is typically aligned with main axes of a scanner, designated axes x and y, and a scanning motion is obtained by moving an ultrasonic probe along one axis or along a combination of axes (for instance x) and repeated after indexing along a perpendicular direction by moving one or a set of axes (for instance y), while maintaining a distance between the probe and the structure surface. Some systems use a plurality of such probes. An example of a method and apparatus for scanning an object is described in Patent Application No. CA 2,820,732, entitled “Method and Apparatus for Scanning an Object”, to Grimard et al., filed on Jun. 27, 2013, the disclosure of which is incorporated by reference herein.
Trigger signals are generated to initiate ultrasonic pulse generation and data acquisition when the probe reaches positions set by the grid. Encoder signals of the main scanning axis (for example x) are monitored by an encoder counter that generates the trigger signals at required probe positions. The probe is indexed in a perpendicular direction by a distance dictated by dimensions of the pixels on the grid at the end of the scanning motion. The scanning motion is then repeated for a new index position.
The ultrasonic signal acquired at each position of the grid is amplified and filtered by a receiving apparatus using fixed filter and amplification settings. A selected time interval of this outputted signal is converted to digital data using an analog to digital converter apparatus. This digital data is recorded on a computer and the amplitude of the signal within a selected time interval (for instance the maximum absolute amplitude) or the time of occurrence of an echo within a selected time interval is associated to each position of the grid. An image named the C-scan is produced by associating colors of a chart to the values associated with each pixel of the grid.
Flaws are typically detected by analyzing the maximum amplitude of an ultrasonic signal within a selected time interval containing an ultrasonic echo that interacted with the test object. In the specific case of through transmission inspection, the echo that travelled from the emitting probe to the receiving probe and through the thickness of the tested object is monitored. Flaws are identified by comparison of the amplitude of one or a group of grid pixels with the amplitude of the surrounding pixels. An increase or decrease of signal amplitude exceeding a level obtained by calibration of the scanning apparatus on calibrated flaws, for example flat bottom holes, reveals the presence of a flaw.
The amplitude of an ultrasonic echo is a function of the presence of flaws, but also of variables associated with the object geometry and composition. Examples of variables that affect the amplitude of an ultrasonic echo interacting with an object are attenuation, diffraction, scattering, as well as changes of mechanical impedance. These variables can have various causes such as, for example, variations of thickness, surface curvature, material composition or the anisotropy of the material.
The amplifier gain of the receiver apparatus is set at a fixed value that allows monitoring the echo and the variations of amplitude of the echo in the presence of a flaw with an acceptable signal-to-noise ratio based on calibration.
For the example of a signal acquired by transmission of ultrasounds through a material, the amplifier gain must be set at a high enough level to monitor the expected variations of amplitude in the presence of a flaw, but low enough to avoid saturation of the echo by the receiver output limits or the acquisition input limits. In addition, the ratio of the signal amplitude recorded on a flawless area to the electronic noise level (i.e. the signal to noise ratio) must be higher than the expected signal amplitude loss due to a flaw.
Identification of flaws in objects of arbitrary shape, thickness and material composition is complicated by variations of echo amplitude naturally occurring within the tested material. If a sound material presents large differences in attenuation, the optimal receiver amplifier gain changes as a function of the attenuation. If the variation of attenuation between two areas of a sound object is too high, it can be impossible to set a unique receiver amplifier gain value that allows evaluating the condition of all areas of the object with the proper signal-to-noise ratio.
If the inspection of an object can be achieved with a fixed amplifier gain, data analysis must be performed with an a priori knowledge of the natural echo amplitude variations of the tested object in order to properly separate signal losses caused by flaws from natural signal amplitude variations.
Sizing of flaws is also complicated by variations of echo amplitude that are not related to flaw. Typical image flaw sizing methods such as −6 dB drop sizing or other C-scan based sizing that make use of echo amplitude require a constant amplitude reference to be reliable.
If the inspection of an object cannot be achieved with a fixed amplifier gain, current practice is to inspect different areas of the object individually at their optimal receiver gain or, when the geometry of the object allows it, to pause the scanning process and manually change the amplifier gain when the probe enters an area that required a different amplifier gain. In the case of an arbitrary object, both alternatives are impractical.
Therefore, there is a need for techniques that enable efficient scanning of test objects having complex geometries.